Earth may be able to create its own water without help from comets

Earth stands out from the rest of the solar system in a number of ways, but most relevant to our continued existence is that it has large quantities of liquid water on the surface. How it all got there is still up for debate. Scientists have long thought that most of Earth’s water was deposited by comets, but recent findings from the ESA Rosetta mission call that into questions. Researchers from Ohio State University have a different hypothesis. What if Earth can make its own water?

This interesting notion comes from associate professor Wendy Panero and graduate student Jeff Pigott. They suggest that previously unknown geochemical processes can produce water deep in the Earth, where it is eventually released into the ocean by tectonic plate movement. If they’re right, this could account for most of Earth’s water, meaning comets and other space objects would have only needed to contribute a little bit.

A few months ago this might have seemed a little less relevant to the discussion, but the data from asteroid 67P/Churyumov–Gerasimenko has called the traditional thinking into question. Rosetta has sampled the water being released by the comet and compared it to the water here on Earth. If Jupiter family comets like 67P were really the source of water on Earth, they should have similar ratios of hydrogen to deuterium (an isotope of hydrogen). Analysis shows that 67P has significantly more deuterium, meaning these comets might not have been responsible. Astronomers have suggested that asteroids might have been responsible, even though they have much less water.

You don’t need to rely on any of that for the process outlined in new research from OSU. The central theme here is that a rock might appear to be dry, and it is for all intents and purposes. However, under the right circumstances, some mineral deposits could produce quite a lot of water. The researchers say that hydrogen atoms can be trapped in the natural voids and crystal imperfections of rocks. Many minerals also have plentiful oxygen. With the right combination of heat and pressure, these atoms can be released to form water.

To test this hypothesis, the team compressed different minerals using a device called a diamond anvil cell. It places small samples of material under enormous pressure similar to what would be experienced deep in the Earth. At the same time, a laser heats the rock to simulate temperatures near the mantle. This provides data on how much hydrogen could be stored by various minerals, allowing them to calculate how much water could be produced at various levels.

Some minerals were found to capture very little hydrogen, but ringwoodite (seen at the top) seems to be a good candidate for subsurface water production. There’s plenty of it down there, and it could work in concert with the similarly juicy garnet deposits deeper in the Earth to maintain a reservoir 50% the volume of all existing surface water. The natural upwelling of material that drives plate tectonic movement on the surface could carry free hydrogen and oxygen up where it ends up as water and escapes into oceans.